Acoustic filters play an important role in many acoustic applications. Ultrasonic filters are used to condition the ultrasonic signal to be more suitable for the applications of ultrasonic transducers such as PZT transducers and MUT transducers. For example, ultrasonic filters can be used to shape the frequency bandwidth of the transducer, introduce desired phase delay and serve as the matching layer or the backing layer of the transducer. Usually the conventional ultrasonic filters are made of one layer or multiple layers of materials with desired thickness and acoustic properties.
In ultrasound imaging applications (such as ultrasonic nondestructive tests and ultrasonic diagnosis), for example, proper filtering is essential to obtain a good range of resolution. Transducer sensitivity and bandwidth are generally improved by adding single or multiple quarter-wave length matching layers between the transducer and the load medium, and this in turn reduces losses between the transducer and load medium. A matching layer is a thin layer of material placed on the front surface of an ultrasound transducer to improve the transfer of ultrasound into the medium of propagation (e.g. soft tissue). The thickness of the layer is usually equal to one fourth the wavelength of the ultrasound in the matching layer (the so-called quarter-wave matching), and the acoustic impedance is often about the geometric mean of the impedances on each side of the matching layer for effective matching. Multiple matching layers have also been used because using multiple layers with decreasing impedance provides a more gradual transition from the high impedance of the element to the low impedance of the body. In addition to matching layers in the front of the transducer, acoustic backing materials are also used at the back of the transducer to block ultrasound leaking into the substrate.
Conventional acoustic filters having one or more material layers have been used with a variety of ultrasound transducers, including piezoelectric transducers (PZT) and micromachined ultrasonic transducers. An ultrasound transducer performs a chain of energy transformation to realize its function of a transducer. In its receiving mode, the acoustic energy of ultrasound waves propagating in a medium where the transducer is placed is transformed to mechanical energy of a movable part (conventionally a vibrating membrane) in the transducer. The motion of the movable part is then transformed to a detectable electromagnetic (usually electrical) signal. In its transmitter mode, the reverse chain of energy transformation takes place.
Various types of ultrasonic transducers have been developed for transmitting and receiving ultrasound waves. Ultrasonic transducers can operate in a variety of media including liquids, solids and gas. These transducers are commonly used for medical imaging for diagnostics and therapy, biochemical imaging, non-destructive evaluation of materials, sonar, communication, proximity sensors, gas flow measurements, in-situ process monitoring, acoustic microscopy, underwater sensing and imaging, and many others. In addition to discrete ultrasound transducers, ultrasound transducer arrays containing multiple transducers have been also developed. For example, two-dimensional arrays of ultrasound transducers are developed for imaging applications.
Compared to the widely used piezoelectric (PZT) ultrasound transducer, the MUT has advantages in device fabrication method, bandwidth and operation temperature. For example, making arrays of conventional PZT transducers involves dicing and connecting individual piezoelectric elements. This process is fraught with difficulties and high expenses, not to mention the large input impedance mismatch problem presented by such elements to transmit/receiving electronics. In comparison, the micromachining techniques used in fabricating MUTs are much more capable in making such arrays. In terms of performance, the MUT demonstrates a dynamic performance comparable to that of PZT transducers. For these reasons, the MUT is becoming an attractive alternative to the piezoelectric (PZT) ultrasound transducers.
Among the several types of MUTs, the capacitive micromachined ultrasonic transducer (cMUT), which uses electrostatic transducers, is widely used. Other MUTs using piezoelectric (pMUT) and magnetic (mMUT) transducers are also adopted.
In general, both matching layers and backing layers are needed for PZT transducer because the significant mismatch of the acoustic impedance between the PZT material and the medium. One of the major disadvantages of the PZT transducer is the narrow bandwidth. A matching layer and a backing layer are therefore usually needed to improve the bandwidth of a PZT. For cMUT, there is usually less need for such layers, especially the matching layer, because the cMUT have better impedance match with the medium and can often exchange energy with the medium without the match layer. However, even cMUT may still benefit from acoustic filtering designed for other purposes.
In the prior art, the matching layer and backing layer is usually a single or multiple layers of special materials with particular acoustic impedances and precise thicknesses. For multiple layers, different types of materials are often needed for different layers. Often, finding a material with exactly acoustic impedance and coating a layer to the exactly thickness can be a challenge for just a single layer, especially for a high frequency transducers, 2D transducer arrays or IVUS side-view transducers, and much more so for multiple layers of high order filtering.